1. Field of the Invention
This invention relates to a recording sheet to be used for ink jet recording and a method of ink jet recording using the same.
2. Related Art
Ink jet recording is a recording method with which the ink contained in an ink chamber is subjected to fluctuations of pressure through mechanical vibration or displacement of the ink chamber by means of a piezoelectric device to produce ink droplets, which are ejected from nozzles toward a recording sheet that receives them for printing. The technique has been attracting public attention because it enables printing to be affected at low noise level and high speed even in color.
A recording sheet to be used for ink jet recording has to meet, among others, the following requirements.
(1) It should accurately produce an intended ink density once ink droplets are deposited on its surface.
(2) It should absorb ink at an elevated rate.
Since ink droplets reaches its surface at an elevated rate, it has to be capable of temporarily and rapidly absorbing ink in its paper layer. The higher the rate of absorption, the greater can be the printing speed. Thus, a recording sheet normally contains or is coated with an appropriate mixture of highly absorbent filler materials (additives) and hydrophilic resins.
(3) It should have high image resolving power.
The size of an dot formed by ink droplets on the surface of a recording sheet will be twice or three times as large as the diameter of an ink droplet. The image resolving power of a recording sheet is a function of the shape (roundness) and the size of the dot formed by ink droplets on its surface and the shape of the dot is closer to a circle on a coated recording sheet than on a noncoated recording sheet.
(4) Other requirements that have to be met include glossiness, clear hues, no stains due to splashed ink nor blots due to uneven dots, no shrinkage of paper at or around printed areas and water/light resistive recorded images.
While there exist no recording sheets that satisfy all the above listed conditions, coated recording sheets provided with a particular coating layer formed by combining fine silica particles and a water soluble binder agent normally show an excellent ink absorbing capability, a remarkable ink color reproducibility and an ability to produce even and circular dots. Fine silica particles can adsorb and hold pigments contained in ink and therefore impart sharpness and clearness to printed color images. Coated recording sheet provided with a coating layer comprising clay or calcium carbonate are also popular although they may be less ink absorbent, less remarkable for ink color reproduction and less capable of producing sharp and clear images. Any of these recording sheets show a cross section as illustrated in FIG. 2, where a fibrous paper substrate 1 carries a coating layer 2.
However, known recording sheets as described above are accompanied by a number of problems as listed below which are particularly significant when they are employed for color printing using more than one primary color inks.
In a multi-color ink jet recording apparatus using recording sheets, the number of different color inks stored in the system is limited (to, for instance, four or yellow, magenta, cyan and black) and a desired color other than these (e.g., red, blue, green) is produced by appropriately combining any of the inks and applying them sequentially to a particular area of a recording sheet. Consequently, such an area eventually can carry a disproportionally large amount of inks, which can permeate into the paper substrate 1 through the coating layer 2 to swell the former, entailing the problem of shrinkage. When the paper substrate shrinks to produce wrinkles, the surface of the recording sheet is corrugated and the ridges of the corrugation can eventually touch any of the jet nozzles of the print head for ejecting ink of the ink jet recording apparatus arranged very close to the surface of the recording sheet to stain the recording sheet and destruct the nozzles of the print head. Even if only an ink is used for printing, wrinkles can appear on the recording sheet when ink is densely or daubingly applied to its surface.